1. Field of the Invention
The present invention relates to a device for measuring a property of a gyratory compactor device and, more particularly, to a method and apparatus for determining an angle of gyration and/or a pressure in a Superpave gyratory compactor.
2. Description of Related Art
The Strategic Highway Research Program (SHRP) has developed a standard for testing the physical properties of a bituminous asphalt paving mix, using a device known as the Superpave gyratory compactor. The Superpave gyratory compactor produces compacted cylindrical test samples of the asphalt paving mix for determining volumetric and mechanical properties. The compactor simulates the kneading action of the large rollers used to compact asphalt pavement during highway construction. Thus, the compacted samples simulate the density, aggregate orientation, and structural characteristics obtained in an actual roadway when proper construction procedures are used in the placement of the paving mix. Typically, a sample of the asphalt paving mix is placed in an open-ended cylindrical mold, with circular disks or plugs being placed at opposite ends of the mold. The gyratory compactor device applies pressure through these disks to the sample in the mold while gyrating the mold at a specified angle to produce a gyratory kneading action. The Superpave gyratory compactor specifications call for the mold to be gyrated at a compaction angle of 1.25 degrees, operating at 30 rpm, while applying a constant pressure of 600 kPa.
Several manufacturers produce gyratory compactors according to the Superpave gyratory compactor specifications. Gyratory compactors of this type are described for example in the following U.S. Pat. Nos. 5,323,655; 5,456,118; 5,606,133; 5,939,642; 5,817,946; and 6,026,692.
During the compaction process, the gyratory compactor machine records the number of gyratory revolutions, the height of the test sample (or more accurately, the position or movement of the pressure ram) and other parameters. In order to have consistent and reproducible results from one machine to another, each machine must be calibrated. These calibrations include measurement of the mold angle (otherwise referred to herein as the “angle of gyration” or “gyration angle”) and measurement of the pressure applied by the pressure ram. Heretofore, the mold angle calibration has been typically carried out in a static mode, wherein the mold angle is mechanically set and calibrated during the initial assembly phase and not adjusted thereafter unless required. The particular method of adjustment generally differs between different machine designs, but usually involves a complex arrangement of sliding surfaces in the form of, for example, cams, pins and slots, linkages, and/or the like. The mold angle adjustment mechanism thus serves to provide an offset between one end of the mold, held in alignment with the ram axis, and the other end of the mold, which is compelled to move in a circular orbit by the kinematic properties of the machine. However, a calibration carried out in a static mode will not necessarily take into account whether the configuration of the gyratory compactor maintains the mold angle within specifications throughout the entire compaction process. Therefore, the angle of gyration must be occasionally verified. However, the correct angle typically can only be verified by a complete re-calibration of the machine and such a re-calibration is often not practical in the field. As such, re-calibration usually requires that the machine be shipped back to the manufacturer, or requires the presence of a factory technician at the site, each of which may involve a sizeable expenditure of time and money.
In other instances, mechanical-type contact sensors have been employed to determine the mold angle. For example, U.S. Pat. No. 5,817,946 to Brovold describes multiple mold angle transducers, each comprising a spring-biased plunger, wherein the tip of the plunger continuously contacts and rests against the outer surface of the cylinder wall of the mold. The relative displacements of the plungers thereby enable the determination of the mold angle. However, the compaction process is a dynamic process, with the mold being constantly gyrated while pressure is applied. Accordingly, the plungers may be prone to wear or misalignment from contact with the mold during the compaction process, thereby making it difficult to maintain calibration and receive consistent measurements from the transducers. Further, such mechanical displacement-measuring devices may not provide the necessary accuracy for measuring the mold angle in accordance with the SHRP standard. In some instances, accretions on the wall of the mold from sample processing may further decrease the accuracy of the mechanical-type contact sensors. These and other factors can affect the actual mold angle measured and indicated during the compaction process.
In addition, the pressure calibration of the machine has also been typically carried out only in a static mode. For example, a load cell can be temporarily positioned in contact with the pressure ram of the compactor to measure the force applied by the ram. However, a calibration carried out in a static mode will not take into account whether the control system of the gyratory compactor holds the pressure within specifications throughout the entire compaction process. The compaction process is a dynamic process, with the mold being constantly gyrated while pressure is applied. Also, the sample within the mold is reduced in height as it is compacted. These and other factors can affect the actual pressure applied to the sample by the gyratory compactor.
Thus, there exists a need for an apparatus, system, and method capable of statically and dynamically determining and indicating the angle of gyration of a mold as well as the pressure exerted on a sample contained therein so as to provide a consistent and readily calibrated mechanism for verifying compliance with the SHRP standard for testing the physical properties of a bituminous asphalt paving mix using a Superpave gyratory compactor, as well as other improvements over existing devices.